The South Korean military fired hundreds of warning shots at a Russian military aircraft that entered South Korean airspace this morning.
Defence officials in Seoul also said Chinese military aircraft entered South Korean airspace.
It was the first time a Russian military aircraft had violated South Korean airspace, an official at the Ministry of National Defence in Seoul said.
The warplane was one of three Russian aircraft to enter the Korea Air Defence Identification Zone (KADIZ) early this morning, one of which entered the zone twice, the defence ministry said in a statement.
Two Chinese military aircraft also entered the KADIZ, it said.
Russia's Ministry of Defence denied that its strategic bombers had violated South Korean airspace.
It accused South Korean jets of carrying out dangerous manoeuvres that threatened its aircraft, the RIA news agency reported.
China's foreign ministry said South Korea's airspace identification zone was not a territorial airspace and all countries enjoy freedom of movement there.
The South Korean government would lodge official complaints about the violations with China and Russia, the defence ministry in Seoul said.
According to the South Korean military, a single Russian aircraft also later violated South Korean airspace over Dokdo - an island that is occupied by South Korea and also claimed by Japan, which calls it Takeshima - just after 9am (1am Irish time).
The Russian aircraft was an A-50 airborne early warning and control aircraft, an official at South Korea's Joint Chiefs of Staff (JCS) said.
South Korea scrambled F-15 and F-16 fighter jets in response to the intrusions.
The South Korean jets fired around 360 rounds of ammunition during the incident, the JCS official said.
"The South Korean military took tactical action including dropping flares and firing warning shots," the defence ministry said.
A South Korean defence official told Reuters that the Russian aircraft did not respond in any threatening way.
The Russian aircraft left South Korean air space, but then entered it again about 20 minutes later, prompting the South Koreans to fire more warning shots.
The ministry said South Korean warplanes "conducted a normal response" to the incursion, without giving further details.